The $20 billion total so far could go even higher. JPMorgan Chase (Charts, Fortune 500), which is set to deliver quarterly results this month, has yet to reveal how it has been impacted by the credit crisis.

In most instances, the writedowns originated from securities backed by subprime mortgages that turned toxic after homeowners began defaulting on their loan payments. To make matters worse, banks got stuck with loans that companies agreed to take when dealmaking was still robust on Wall Street.

But the mood among bank CEOs has been one of optimism. Executives from firms like Merrill and Citi have said that other areas of their businesses continue to perform well and that there have been signs that credit conditions are improving.